Plioto-Electric Theory of Vision. 35 1 



evidence in favour of the photo-electric theory of vision, 

 but, on the other hand, it certainly would not be justifiable 

 to conclude from them that the materials tested have no 

 photo-electric properties, since the difficulties of getting 

 intimate contact with the sensitive film are, as already 

 mentioned, probably quite sufficient to account for the nul 

 results obtained. 



After the failure of these experiments to show any 

 positive effects, a direct method of testing the black pigment 

 etc. was used. 



The usual plan of doing this was employed. The material 

 to be tested is placed on an insulated conducting plate, and 

 the rate of leak of negative electricity from the plate when it 

 is illuminated measured with an electrometer. To facilitate 



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the escape of the negative electrons, an accelerating electric 

 field is generally used. A sketch of the various electrical 

 connexions is given. The Wilson Kaye Tilted Leaf Electro- 

 meter was used to measure the electronic current. Its high 

 sensibility to voltage changes and small capacity render it 

 peculiarly suitable for this purpose. Fortunately it was 

 found unnecessary to work with the instrument in its most 

 sensitive condition, as the natural leak in the ionization- 

 chamber limits the sensitivity at which it is desirable to 

 work. Usually it was fount! that a sensitivity of about 

 30 or 40 scale-divisions to the volt was sufficient, though 



